There are different physical principles enabling the contactless locating of a mobile object in space. One of the systems commonly in use involves an electromagnetic emitter placed in a known fixed point emitting electromagnetic fields of known distribution in different directions, these fields being detected by an electromagnetic sensor placed on the mobile object. These fields can be used to determine the position of the object in relation to the emitter. The invention is intended to improve a device emitting electromagnetic fields in precise directions in relation to a reference system X, Y and Z.
Conventionally, electromagnetic fields are emitted by controlling an alternating current at a given frequency in a winding using a closed loop. In all implementations, the complete coil has three windings, one for each geometric axis X, Y and Z. Each winding comprises two half-windings, the terminal voltage of the entire winding being symmetrical with respect to ground, in order to minimize the electric field emitted. The problem of power supply and electrical control of the coils is not so simple.
FIG. 1 shows the principle of the first known implementations of electrical control along an emission axis, the amplification for each axis being differential. On this and the following figures, the conventions concerning representation of electronic components and functions have been used.
If a class AB linear amplifier 2 is used, the resonance effect is normally used to limit the draw of the device. This resonance ensures that the terminal voltage of the half-windings 1 can be much higher than the power supply voltage VP of the amplifier. To emit on the three axes X, Y and Z, the electronic control device comprises three electronic assemblies identical to the one in FIG. 1. In the assembly in FIG. 1, the higher the quality factor of the tuned circuit rises, the greater the voltage gain becomes. However, in this case, selectivity is increased. As dispersion and drift in the different components needs to be taken into account, a compromise is required. A quality factor of about 5 to 10 may be appropriate. Tuning is effected for a given emission frequency. To change the frequency, the value of certain components must be changed to centre the resonance on this new frequency, which creates a first difficulty. Another significant technical difficulty relates to coupling between the axes. Even if the geometry of the winding is optimized, undesirable couplings persist, primarily a mutual inductance between the windings of different—theoretically orthogonal—axes.
The circuit diagram in FIG. 1 suggests that the current in each axis, being closed-loop controlled by an amplifier as a result of the use of a high loop gain, can be forced to be the exact image of the setpoint excitation. In reality, this solution is not possible. An open-loop gain greater than 103 would be required at the emission frequency. To satisfy the stability conditions of the loop, the bandwidth thereof would be very high, reaching tens of MHz. Consequently, the amplifier would require a gain-bandwidth product of several tens of GHz. Even in the case of a gain-bandwidth product and not a bandwidth, it cannot be implemented, because the device would become much too sensitive to internal external disturbances related to the parasitic elements of the winding, the cabling to connect the amplifier to the coil, and thermal noises of the electronic components.
To address the coupling problems, the patent FR 2 685 491 discloses an emitter control system that only emits on a single axis at a time. The device is based on four alternating phases: emission X, then emission Y, then emission Z and finally a self-calibration phase. When emitting on one axis, the circuits of the windings of the two other axes are forced open. Consequently, no mutual-inductance currents can pass through these windings. A schematic diagram of the existing device is shown in FIG. 2. As shown on this figure, the open or closed position of the switches 4 enables a single winding to be powered. Advantageously, this device takes advantage of the use of transformers 3. Consequently, a single amplifier 2 is used. The power supply symmetry of the windings 1 is provided by the secondary of the transformers. The winding circuits are opened using MOSFETs used on switches 4, the crosstalk between axes being minimized by short-circuiting the transformers using switches 5. The voltage to be applied to each coil can be optimized by adjusting the transformation ratios. However, the existing device becomes more complex once the emission frequency needs to be switched. This requires the addition of electromagnetic relays to switch the windings on the transformers. In short, the drawbacks of this device are:                Inability to program a specific frequency from a large number of possible choices;        Alternating emission, one axis after the other, which reduces the integration time for the measurement;        Volume of the emission electronics on account of the transformers and relays used.        
A solution for eliminating tuned-resonance amplification and obviating the need for large transformers is to use chopper-stabilized amplifiers. The use of three differential PWM amplifiers is not less efficient in terms of volume than the existing solution. However, building low-crosstalk switches using MOSFETs to interrupt the current in windings is theoretically very complex. The problem of emission frequency flexibility is resolved, but this requires continuing to emit alternately, on just one axis at a time.